The present invention relates to the distribution of a time and frequency reference to radio base stations. More specifically, the present invention relates to an efficient way to distribute a frequency and time reference between nodes in a packet-switched network.
In wireless communication systems, such as cellular telecommunications systems, radio base stations are typically interconnected by a packet-switched network. Radio base stations typically use a phase-lock loop, including a voltage controlled oscillator, to maintain frequency stability of the signals generated by the radio base stations. In conventional wireless systems, frequency stability of an oscillator in a radio base station is maintained using a frequency reference which is distributed to radio base stations over the packet-switched network using the pulse code modulated (PCM) reference, i.e. the frame synchronization signal in the E1/T1 transmission protocol. For a more detailed explanation of PCM reference signals, the reader should refer to U.S. Pat. No. 5,870,001 to Österling et al., which is herein expressly incorporated by reference. The use of a frequency reference ensures that the downlink signal generated by the voltage controlled oscillator is properly transmitted to a mobile terminal without interfering with other concurrently transmitted downlink signals, i.e., that the radio base station is transmitting to the mobile station over the correct frequency.
As the sophistication of networking technology increases and the cost of implementation of these networking technologies decreases, radio base stations may soon be interconnected together using newer types of networks. These newer types of networks may not necessarily contain the PCM reference which is present in the E1/T1 transmission protocol. Since these networks may not contain the PCM reference, a new method for correcting the frequency drift of base station oscillators is needed.
Conventional networking technologies use time stamp algorithms to distribute time. In TCP/IP based networks, the network time protocol is used to exchange time stamp messages between network nodes. However, the network time protocol only provides for the distribution of time and not the distribution of frequency as would be required for a radio base station. Further, the network time protocol considers all time stamp messages as equally accurate. Hence, the network time protocol does not take into consideration whether a transmitting node has a very accurate oscillator, e.g., a GPS receiver, or if the transmitting node has an inaccurate oscillator, e.g., a short-term stable free-running oscillator which has not been tuned for a long time.
In addition, the network time protocol does not allow a node to receive frequency information from one node and time information from another. Further, network time protocol is designed for time stamping on an application layer which results in large uncertainties as to the accuracy of the time stamp.
Since, radio base stations may soon be connected to each other through many other types of networks, e.g. ethernet networks, a new method for correcting the frequency drift of oscillators in radio base stations is required. Accordingly, it would be desirable to provide a reference frequency for radio base stations in networks which do not have a PCM reference signal. Further, it would be desirable to provide a method and apparatus for a node in a network to determine the accuracy of its oscillator.